Remove replacement selection sort

At the time replacement_sort_tuples was introduced, there were still cases where replacement selection sort noticeably outperformed using quicksort even for the first run. However, those cases seem to have evaporated as a result of further improvements made since that time (and perhaps also advances in CPU technology). So remove replacement selection and the controlling GUC entirely. This makes tuplesort.c noticeably simpler and probably paves the way for further optimizations someone might want to do later.